Most sectional doors employ a track system that guides the door panels from a closed generally vertical position to an open generally horizontal position. Sectional doors normally employ a counterbalance system having a spring to counteract the weight of the door. Optimally, the counterbalance spring would have sufficient tension, such that the door would fully close and only a small amount of force would be necessary to raise the door from the closed position. Counterbalancing springs can be either extension springs or torsion springs. Torsion spring counterbalance systems normally have an axle to distribute the torsional force from the counterbalance spring or springs equally to the door through cable storage drums and cables attached thereto. The counterbalance springs can either be enclosed within the axle or placed outside an axle. The counterbalance springs are pre-tensioned during installation.
Safety considerations dictate the need for a device that prevents the door from falling in the event of eventual failure of one of the components of the counterbalance system, for example, the rupture of the counterbalance spring. Failure of the counterbalance spring while the door is partially open can result in the unexpected and uncontrolled closure of the door. This may cause inconvenience to the user, as well as present safety concerns to anyone located underneath the door at the moment of spring failure. Consequently anti-drop assemblies have been developed that, upon failure of the counterbalance assembly, stop or at least slow descent of the door.
One type of anti-drop mechanism senses loss of tension in the counterbalance cable. The counterbalance cable attaches to the cable drum which, using force of the counterbalance spring and a driving force, raises the door to the upright position. If during operation the cable breaks, the cable in turn loses tension. An example of this type of anti-drop mechanism has a pawl that engages the cable during normal operation, and upon loss in tension, a bias member overcomes any remaining cable tension and pushes the pawl into contact with a stop surface to stop descent of the door. This mechanism is effective against cable breaks but does not protect against counterbalance spring failure.
In another type of anti-drop system, centrifugal force is used to brake the door upon counterbalance spring failure. Typically such systems include a stator, a rotor and a stop dog held in place by bias elements. If the counterbalance spring breaks, the door drops at speeds that are faster than normal operation. As the rotor spins, the centrifugal force overcomes the bias force and moves the stop dog into contact with the stator, thereby stopping the rotor. The reaction speed of these systems is slower than other safety systems, allowing the door to fall further and thereby increase the chance of damage or injury. Further, stronger components are normally needed for such systems, increasing the cost and complexity of the door control components.
Various other anti-drop systems employ arrangements where a counterbalance spring or its axle is attached to a ratchet wheel or a pawl such that upon loss of spring tension the pawl engages the ratchet wheel to lock the mechanism that effects raising and lowering of the door or other barrier. These systems are typically relatively complex in terms of the number and design of the component parts. Further, this complexity and the probability of deterioration or corrosion over years in an inoperative status makes operation of the system problematic when the counterbalance spring normally fails. In addition, many of these anti-drop systems are limited by their design to utilization with counterbalance systems wherein the torsion springs are mounted outside the axle.